Diffraction grating beam splitter

ABSTRACT

The optical system divides a beam of coherent radiation into a pair of beams having powers according to a desired ratio. The primary beam is intercepted by a holographic diffraction grating positioned at approximately the Bragg angle, placing virtually all the diffracted energy into the 0th and the selected one of either the +1st or -1st diffraction orders. The ratio of the power of the 0th order to selected one of the +1st or -1st diffraction order is varied by changing the angle of the grating with respect to the primary beam. The ratio can be set by calibrating the angle of the grating in terms of the split ratio or by monitoring the power of the unselected one of the +1st or -1st diffraction order. The output signal from the power monitor in the unselected order can also be used to control the power of the primary beam.

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United States Dickson 1 Oct. 23, 1973 1 DIFFRACTION GRATING BEAMSPLITTER [57] ABSTRACT 1 lnvemor? Q Y David Dicks, Rochester, Theoptical system divides a beam of coherent radiation into a pair of beamshaving powers according to a [73] Assignce'. International BusinessMachines desired ratio' The Primary beam is intercepted by aCorporation, Armonk, holographic diffraction grating positioned atapproximately the Bragg angle, placing virtuall all the dify 1 Filed!June 1972 fracted energy into the 0th and the selected one of ei- [21]Appl No: 265,847 ther the +lst or lst diffraction orders.

The ratio of the power of the 0th order to selected one of the +lst orlst diffraction order is varied by [52] 356/218 changing the angle ofthe grating with respect to the 51 I t Cl 1/42 GOZb 27/00 primary beam.The ratio can be set by calibrating the 2 1 225 226 angle of the gratingin terms of the split ratio or by l 1 le 0 can 350/3 5 monitoring thepower of the unselected one of the +lst or lst diffraction order. Theoutput signal from the power monitor in the unselected order can also be[56] uNlTE lg s zi r E zs giiENTs used to control the power of theprimary beam.

3,611,181 10/1971 Lary et al BSD/162R Primary Examiner-David SchonbergAssistant Examiner-V. P. McGraw Att0rneyCarl W. Laumann, Jr. et a1.

10 Claims, 1 Drawing Figure an w 3515/218- 1 DIFFRACTION GRATING BEAMSPLITTER BACKGROUND OF THE INVENTION DESCRIPTION OF THE PRIOR ART Forthe generation of holograms having optimum characteristics it isessential to control the ratio of the power of the light from the objectto the power of the light in the reference beam. Traditionally this hasbeen accomplished by a reflective type of variable ratio beam splitter.Typical of such beam splitters is a device having a metallic filmdeposited on a glass plate. The thickness of the film, and therefore thereflectivity, varies over the surface of the plate. When the plate iscircular in form and mounted for rotation so that areas of variousreflectivities can be introduced into the beam, it is possible to obtainthe desired ratios between the transmitted and the reflected beam.

This arrangement is fully acceptable in certain situations, but is doesnot have disadvantages which make it less than ideal. For example, theratio of transmitted to reflected light is dependent upon the plane ofpolarization of the light coming from the laser source. This isparticularly true when the splitter is arranged so that the amount ofreflected light is low. In this case, the splitter acts like adielectric instead of a metal and the ratio of reflected to transmittedpower will be depedent upon angle of incidence, refractive index of thedielectric and polarization of the incident light, according to FresnelsLaws of Reflection.

The use of this type of beam splitter usually results in an angle of 90or more between the transmitted and the reflected beam. This isundesirable in the situation where space limitations prohibit folding ofoptical paths for path length equalization.

In addition to those shortcomings, the reflective splitter suffers fromthe characteristic that the total light which is contained in the tworesulting beams will vary as the split ratio changes. The minimum totalenergy in the two resulting beam occurs when the split ratio is 1:1because the scattering and absorption of energy by the metallic film isthe maximum at this point. Approximately one half of the beam power islost when the split ratio is 1:1. The loss ofpower is not a realproblem, however, the amount of power which is lost can varyconsiderably as the split ratio is changed.

Because the power in the two resulting beams is a variable quantity, itis necessary to measure the power in a beam after a change in the splitratio is made. Without such a measurement, the correct exposure time canonly be estimated, since the amount of energy in the two beams isunknown.

With this type of beam splitter, it is difficult to continuously monitorthe laser output without obstructing the primary beam or the tworesulting beams. Where the width of the beam exceeds that required forthe particular use, the excess width of the beam can be intercepted formonitoring purposes. Systems with a constant beam power generallyinclude means for monitoring the beam so that variations in laser outputmay be corrected by changing the input power to the laser. Placing amonitor in the periphery of the beam is usually a cumbersome solution.Each time the beam is altered, the monitor must be repositioned andcalibrated once again.

SUMMARY OF THE INVENTION A holographic Bragg phase diffraction gratingis used LII as the beam splitting element. The grating may be fabricatedin accordance with the technique described in Applied Optics, Vol. 9,No. 7, July 1970, p. 1943. The grating is positioned on a rotatablemount in a location to intercept the primary beam at an angle whichapproximates the Bragg angle. Approximately 50 percent of the primarybeam is absorbed or scattered. For angles close to the Bragg angle, 50percent of the power of the primary beam is transmitted into the 0th and+lst diffraction order. A small amount, 0.02 percent, appears in the-lst diffraction order.

As the grating is rotated to a new angle, the total power in the +lstand 0th orders remains essentially constant; however, the ratio of thepower in the +lst order to that in the 0th order changes. A small, butmeasurable, change of the power in the -lst order allows the split ratiobetween the 0th and +lst orders to be indirectly monitored.

The output of the monitor can be used to determine the proper gratingangle for a desired split ratio without the need for measurements of the0th an +lst diffraction orders. The output of the monitor can be used toregulate the laser power to a constant value once the grating is at theproper angle.

It is therefore an object of this invention to provide an improved beamsplitter for coherent radiation.

It is a further object of this invention to provide a beam splitterwhich provides a variable ratio while holding the total power in theresulting beams at an essentially constant value.

It is a further object of this invention to provide a beam splitterwhich allows indirect monitoring and regulation of beam power in theprimary beams.

Still another object of this invention is to provide a beam splitterwhich is relatively insensitive to the plane of polarization of theincident beam.

Still another object of this invention is to provide a beam splitter inwhich the emergent beams are separated by an angle less than Theforegoing and other objects, features and advantages of the inventionwill be apparent from the following more particular description ofpreferred embodiments of the invention as illustrated in theaccompanying drawing.

DESCRIPTION A diffraction grating l is positioned to intercept theoutput beam 2 from laser 3. Grating 1 is mounted on a rotatable shaft 4so that the angle 0, which is the angle between the incident beam 2 andgrating 1, may be varied. Shaft 4 is affixed to the center of rotarytable 5. A drive shaft 6 allows table 5 to be driven in either aclockwise or counterclockwise direction by motor 7. The rotary table 5may be of the general type sold by the J. A. Noll Co., Monroeville, Pa.

When it is positioned as shown, grating 1 will be ef- I fective todivide the incident primary beam 2 into three beams, the 0th order 15,the +lst diffraction order 16 and the lst diffraction order 17. Grating1 will absorb some of the power of the incident beam and scattering willcause further power to be diverted from resulting beams 16, 17 and 18.When grating l is fabricated according to the fashion described in theApplied Optics article and operated near the Bragg angle, approximately50 percent of the power is absorbed or scattered by the grating. Aminute but measurable portion is diffracted into the 1st diffractionorder. The remainder,

which is approximately 50 percent of the power of the incident beam, isdiffracted into the th and +lst diffraction orders.

An object 20 is positioned in the path of the 0th order. A recordingplate 21 is positioned in the path of the +lst order. Although beamexpanding lenses are not shown, it will be appreciated that suchcomponents may be added to the system where the nature and size of theobject 20 requires it. Some of the light diffracted by object 20 reachesplate 21 to form an interference pattern with the +lst order beam. Thisarrangement is capable of producing a hologram on plate 21 provided thatthe ratio of the intensity in the +lst order beam, at plate 21, to theintensity in the diffracted beam from object 20, at plate 21, isproperly set. For optimum holograms, this ratio is set to approximately4:1. That is, the intensity in the reference beam at plate 21 (+lstorder 16) is four times the intensity in the beam diffracted by object20, at plate 21. The desired split ratio can be set by rotating grating1 to vary the angle between the incident beam 2 and the grating.

Grating 1 is a holographic Bragg phase grating. The preferred device isa high efficiency phase diffraction grating constructed according to thebleached hologram process described on page 48, in Principles of H0-lography, H. M. Smith, 1969, Wiley & Sons. The grating is constructed ina large angle, or more, interferometric system.

A grating of this type exhibits the characteristic that only a singlediffraction order (lst) is produced when the grating is illuminated by abeam of coherent light. The amount of light diffracted into the +lstorder is determined by the angle 0 at which the incident beam strikesthe grating. When the angle 0 is equal to the Bragg angle, the power inthe +1 st order beam is a maximum. As the grating is rotated away fromthe Bragg angle, the power of the +lst order beam decreases and thepower in the 0th order beam increases, but the sum of the power in thetwo beams remains essentially constant.

There is a small amount of power diffracted into the lst order. Theactual amount is dependent on the angle 0. The value is a minimum at theBragg angle, typically l/l ,000 of the sum of the power in the 0th and+lst order beams. As the grating is rotated to increase the angle 0,there is less power diffracted into the +lst order and more power in the0th order. The power in the lst order bears a definite relationship tothe angle 0 and therefore also the split ratio between the +lst orderand the 0th order.

From the chart below, it can be seen that the power in the lst orderrises quite rapidly as the angle is increased from the Bragg angle. Thevalue peaks at a split ratio of approximately 5, although the angle atwhich this peak value occurs will vary somewhat depending on the actualBragg angle and the emulsion thickness of the film. Further increase inthe angle 0 results in a decrease in the amount of power in the lstorder.

Angle Split Power Power Power Power Ratio 9' ol -n o 1 ad- +1 i (m (m (m(M Since there are two split ratios for most values of power in the lstorder, it is not possible to determine which of the two split ratiosexists by measuring the power in the lst order alone. In actualpractice, this is not a problem because holography generally requiresthe higher range of split ratios, 5:1 and upward, to the exclusion ofthe low ratios. In interferometry, the general, requirement is for lowersplit ratios, for example 1:1 through 521. Depending on the type of workbeing done, one range or the other can be selected. Mechanical stops canbe put on the table 5 to limit the movement to the device range.

Referring again to the drawing, a sensitive monitor such as light meter30 such as the LITE MIKE manufactured by E. G. & 6., Inc. (Edgerton,Germeshausen & Grier) is positioned to monitor the power in the 1 storder beam. In the event that it is desired to hold the output power ofthe laser 3 at a constant predetermined value, switch 31 is positionedto supply the output signal on from monitor 30 on line 32 to the input33 of comparator amplifier 34. Input 35 is energized from potentiometer36 which has a constant reference voltage applied to terminal 37. Anoutput signal from comparator amplifier 34, on line 38 indicates thatthe signal on line 35 is greater than the signal on line 33. An outputsignal on line 39 indicates that the signal on line 33 is greater thanthe signal on line 35. The output signals on lines 38 and 39 thereforerepresents the result of the comparison between the actual power leveldetected by monitor 30 and the desired power level as established by thesetting of potentiometer 36. The output signals from comparatoramplifier 38 are applied to the power supply 40 for the laser 3.

Power supply 40 responds to the signal on line 38 by decreasing itsoutput power to reduce the output power of laser 3 to a point where thesignal on lines 33 and 35 are equal. A signal on line 39 causes anincrease in the output power of laser 3. By properly adjusting the gainand feedback values of th various stages in comparator amplifier 34 andpower supply 40, it is possible to hold the power level in the +lst and0th orders to a constant level without monitoring these orders directly.

While the drawing shows control of the laser output power as the meansfor regulating to power in the selected order, it is possible to use anunregulated laser and achieve control by means of a modulator in theprimary beam 2.

The ability to monitor the split ratio can be utilized as a convenientmeans for automatically setting the split ratio from a remote position.When this mode of operation is desired, switch 31 is set to thealternative position to connect the monitor output signal on line 32 toline 47. The other input terminal 46 of comparator amplifier 47. Theother input terminal 48 is connected to split ratio potentiometer 50which is energized from a voltage reference source connected to terminal51.

Comparator amplifier 47 has a pair of output terminals and 56. An outputsignal at terminal 55 indicates that the signal at terminal 46, from thepower monitor 30, is greater than the signal at terminal 48 from thesplit ratio potentiometer 50. Similarly, an output signal at terminal 56indicates that the signal at terminal 48 is greater than the inputsignal at terminal 46. There is a dead-band where the signals atterminals 46 and 48 are very nearly equal and no signal exists at eitheroutput terminals 55 or terminal 56.

The selector switch 60 is shown in the position where it connects thesignals on terminals 55 and 56 to the input terminals 70 and 71 of motordrive amplifier 72. A pair of leads 78 and 79 connect the output ofmotor drive amplifier 72 to motor 7. In the embodiment .shown, motor 7is a d.c. motor which can be driven in either direction depending on thepolarity of the voltage applied across leads 78 and 79.

An input signal at terminal 70 indicates that the power level in the lstorder is greater than the reference signal from potentiometer 50. Inthis case, motor drive amplifier 72 will generate an appropriate voltageacross leads 78 and 79 to cause motor 7 to rotate in the direction whichwill change the angle of grating 1 to reduce the magnitude of the powerin the lst order. If the system is operating in the region where thesplit ration varies from 1.421 to 5:1. the grating 1 will be rotated toreduce the angle, thereby placing more power in the +1st order and lesspower in the 0th and lst orders. Rotation of grating 1 would be in theclockwise direction until the power in the lstorder decreases to thepoint where the signal at terminal 46 is equal to the signal at terminal48.

When operating in this mode, the output power of the laser 3 must beheld constant since variations in laser output would be interpreted aschanges in the reference signal from potentiometer 50. The usual mode ofoperation would be to use this mode only for the short period of timerequired to change the position of grating 1 to a new angle. The normalsequence would be to set the potentiometer 50 to a position whichindicated the desired split ratio. Switch 31 would then be operated toconnect the output signal on line 32 to line 45. Switch 60 would then bemoved to the position shown in the drawing and motor 7 would beginoperation. When motion of the grating to the new position was complete,switch 60 would be moved to disconnect the inputs to motor driveamplifier 72. Switch 31 would very likely be changed back to theposition shown so as to again regulate the output power of laser 3.

Since the relationship between the power in the 0th order and the +1 storder is also dependent on the angle that the grating presents to beam2, it is possible to adjust the split ratio from a remote location byusing the angular position as a feedback signal. To operate in thismode, switch 60 is positioned so that the input terminals 70 and 71 ofmotor drive amplifier 72 are connected to the output terminals 81 and 82of amplifier I 83. Input terminals 84 and 85 are connected to beenergized by the signals from potentiometers 91 and 92, which are bothenergized from a reference voltage source connected to terminal 93.

In this mode of operation, the slider on potentiometer 91 is set to thepoint on a dial scale which indicates the desired split ratio. Thisscale can 'be derived by a calibration procedure in conventionalfashion. When the output voltage from potentiometer 91, applied to inputterminal 85, is not equal to the voltage applied to input terminal 86from potentiometer 92, a signal will appear at output terminal 81 or 82depending on the relative magnitudes of the input signals. An outputsignal from comparator amplifier 83 will cause motor drive amplifier 72to energize leads 78 and 79 with an appropriate polarity voltage todrive motor 7 in the correct direction. When motor 7 rotates shaft 6, amechanical connection represented by dashed line 95 causes the tap onpotentiometer 92 to move in the direction which makes the output voltagefrom potentiometer 92 more nearly equal to the input voltage frompotentiometer 91. Motor 7 continues to drive the shaft 6, therebyrotating grating 1, until the output signal from potentiometer 92 isequal to the output signal from potentiometer 91. At this point, the twoinput signals to comparator amplifier 83 are equal and the output signalat terminal 81 or 82 disappears. With no signal at either input terminalor 71, motor drive amplifier 72 ceases to supply a voltage across leads78 and '79 and motor 7 stops rotation.

This system is not sensitive to the plane of polarization of beam 2.There is no need to control or set the plane of polarization when usingthis beam splitter.

Calibration of the system for the various modes of operation can beeasily accomplished. Suitable scales wouldbe affixed to the slidersassociated with potentiometers 36, 50 and 91. The scale on potentiometer36 would be calibrated in units of output power. Potentiometer 50 wouldbear a scale calibrated in spilt ratio, as would potentiometer 91.

Potentiometer 36 can be calibrated in units of output power for eachdesired split ratio or those which are most commonly used. To do this,the grating would be set to provide a given split ratio. A second powermonitor would be placed in the path of beam 15 or 16. With switch 31 inthe position shown in the drawing, the reading of the second powermonitor would be recorded on the scale associated with potentiometer 36.A new position for the slider on potentiometer 36 would be selected andthe power indicated by the second power monitor would be recorded on thescale. These steps or their equivalent, would be repeated untilcalibration is complete.

Calibration of potentiometer 50 requires that the power in the 0th andthe +1st order be measured. This measurement could be performed by asingle power monitor which would be placed first in the path of beam 15and then in the path of beam 16. In the alternative, monitors could beplaced in the path of each beam. Potentiometer 50 would be calibratedwith switch 30 in the position connecting the signal on line 32 to theinput terminal 46 of comparator amplifier 47. Switch 60 would be in theposition shown in the drawmg.

For each position of the slider on potentiometer 50, a reading of thepower in beams 15 and 16 would be made. From these readings, the splitratio would be cal culated and entered on the scale associated withpotentiometer 50. g

While the drawing is illustrative of a system in which the beam splitteris-automatically set in response to the signal from potentiometer 50, itis also possible to set the beam splitter manually by means of theknurled knob 8 on shaft 6. In such a system, th meter on monitor 30would be calibrated by split ratio. This calibration would proceed withswitch 60 in the intermediate position so that no signal is applied toinput terminals 70 and 71. The control of power supply 40 is shiftedfrom the output of comparator amplifier 34 to an interval control whichis independent of the value sensed by monitor 30.

Power monitors would be placed in the path of beams 15 and 16. Theknurled knob 8 would be used to move grating 1. For each discreteposition of grating 1, the power in beams 15 and 16 would be recorded.From these figures, the split beam ratio can be calculated and enteredon the scale of the meter associated with monitor 30.

The calibration of potentiometer 91 would resemble calibration of themeter associated with monitor 30. The control of power supply 40 isshifted from the output of comparator amplifier 34 to an internalcontrol which is independent of the vlaue sensed by monitor 30. Switch60 is in the position connecting output terminals 81 and 82 ofcomparator amplifier 83 to input terminals 70 and 71 of motor driveamplifier 72. Power monitors would be placed in beams l and 16.

The slider on potentiometer 91 would be set to an initial position andthe servo system which includes comparator amplifier 83, motor driveamplifier 72, motor 7 and feedback potentiometer 92 is allowed tosettle. The power in beams and 16 is read from the power monitors andthe split ratio is calculated. This split ratio is then entered on thescale associated with potentiometer 91. The same split ratio can also beentered on scale 100 on the positioning device for grating 1.

The calibration would be performed for a sufficient number of positionsof potentiometer 91 to provide the amount of scale resolutionappropriate for the application.

In the case where the rotary table 5 has a scale calibrated in degrees,it may be convenient to prepare a calibration table which indicates thesplit ratio for given angular positions. Such a table would resemble theforegoing compilation of data with as many additional entries as arerequired to provide the needed resolution.

The device exhibits a sensitivity to grating emulsion thickness and theBragg angle. For gratings with large Bragg angles or thick emulsions,there is a greater sensitivity to angular rotation so that a large rangeof split ratios can be obtained with small rotation. This may bedesirable to reduce the rotational shift of the +lst order. While notobjectionable in most cases, the example shown rotated the +lst order 1for a 5 rotation of the grating. A 2.5 rotation, from 5.5 to 8",provided a range of split ratios from 5:1 to 33:1. A 5 rotation, fromO5, provided a range of split ratios from 1.4:] to 5:1.

in this description, data is provided for angular excursions startingwith the Bragg angle and extending to greater angles up to 8. The sameeffect has been observed for excursions in the opposite direction, thatis, at angles less than the Bragg angle.

While temporal coherance (monochromatacity) is not absolutely necessary,the system is wavelength sensitive and would exhibit some colorspreading.

This slight change in the angle of the +lst order is not objectionablein most cases and could be overcome by using an FM grating where arotational change also served to alter the grating frequency.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. Means for dividing a primary beam of coherent radiation into a pairof beams having power according to a predetermined ratio, said meanscomprising:

a diffraction grating positioned to intercept said primary beam at anangle to place essentially all the energy affected by said grating intothe 0th and the selected one of the +lst or lst diffraction orders, and

power monitoring means positioned to intercept the resulting beam in theunselected one of said +lst or lst diffraction orders to provide anoutput signal representing the relative powers of the beams in the 0thand selected one of said +lst or lst diffraction orders.

2. A means according to claim 1 wherein said diffraction grating is aholographic phase diffraction grating.

3. A means according to claim 2 wherein the angle is approximately theBragg angle.

4. Means for dividing a beam of coherent radiation into a pair of beamshaving powers controlled according to a predetermined ratio, said meanscomprising:

a diffraction grating positioned to intercept said beam at an angle toplace essentially all the energy affected by said grating into the 0thand the selected one of the +lst and the lst diffraction orders,

monitoring means positioned to intercept the resulting beam in theunselected one of said +lst or lst diffraction orders to provide anoutput signal representing the power to said resulting beam, and

positioning means responsive to said output signal for rotating the saiddiffraction grating to set the power of said unselected beam to adesired value whereby the power ratio of the 0th to the one other ofsaid +lst and lst diffraction orders is set to a desired value.

5. In a system for dividing a beam of coherent radiation into a pair ofbeams having a desired power ratio, the combination comprising: 7

a holographic Bragg phase diffraction grating positioned to interceptsaid beam at approximately the Bragg angle to place essentially all thenonabsorbed energy affected by said grating into the 0th and theselected one of the +lst and lst diffraction orders,

means for rotating said grating to vary the angle at which the beam isincident on the grating,

indicating means, coupled to said rotating means for representing theangular position of said grating,

said indicating means having a scale calibrated according to the ratioof power of the 0th and the selected one of the +lst and lst diffractionorders.

6. The method of calibrating a holographic Bragg phase diffractiongrating in terms of the split ratios of power between the 0thdiffraction order and the selected one of the +lst or lst diffractionorders comprising the steps of:

l. positioning said grating to intercept a beam of coherent radiation atapproximately the Bragg angle,

2. measuring and recording the power of the 0th order beam,

3. measuring and recording the power of the selected one of the +lst orlst diffraction order,

4. calculating the split ratio of the power levels recorded in the steps2 and 3, i

5. measuring and recording the angle of said grating to the incidentbeam,

6. repositioning the gratings to a slightly different angle andrepeating steps 2 5 to develop a table of split ratios for gratingangles in the range of the Bragg angle.

7. The method of calibrating a holographic Bragg phase diffractiongrating in terms of the power of the unselected one of the +1 st or lstdiffraction order and its relationship to the split ratio between the thdiffraction order and the selected one of the +lst or lst diffractionorders comprising the steps of:

l. positioning said grating to intercept a beam of coherent radiation atapproximately the Bragg angle,

2. measuring and recording the power of the 0th order beam,

3. measuring and recording the power of the +lst order beam,

4. measuring and recording the power of the lst order beam,

5. repositioning the grating to a slightly different angle and repeatingsteps 2, 3 and 4,

6. repeating the preceding steps to provide sufficient measurements tocalibrate the split ratio between the 0th order and the selected one ofthe +lst or -lst order in terms of the power of the unselected one ofthe +lst or the -lst order.

8. In an optical system for dividing a primary beam of coherentradiation into a pair of beams, means for holding the power of saidprimary beam to a constant value comprising:

a diffraction grating positioned to intercept said priid mary beam at anangle to place essentially all the energy affected by the grating intothe 0th and the selected one of the +lst and lst diffraction orders,

monitoring means positioned to intercept the resulting beam in theunselected one of said +lst or lst diffraction orders to provide anoutput signal representing the power of said beam,

output power control means for varying the power of said primary beam inresponse to a signal applied to a control terminal,

means connecting said output signal from said monitoring means to saidcontrol terminal whereby a change in the power in said primary beamresults in a change in the output signal from said monitoring meanswhich is effective to restore the beam to the original power levelthrough said control means.

9. A system according to claim 5 wherein said output power control meansincludes a power supply for the source of the primary beam.

10. A system according to claim 5 wherein said output power controlmeans includes a modulator positioned to intercept said primary beam.

1. Means for dividing a primary beam of coherent radiation into a pairof beams having power according to a predetermined ratio, said meanscomprising: a diffraction grating positioned to intercept said primarybeam at an angle to place essentially all the energy affected by saidgrating into the 0th and the selected one of the +1st or 1st diffractionorders, and power monitoring means positioned to intercept the resultingbeam in the unselected one of said +1st or -1st diffraction orders toprovide an output signal representing the relative powers of the beamsin the 0th and selected one of said +1st or -1st diffraction orders. 2.A means according to claim 1 wherein said diffraction grating is aholographic phase diffraction grating.
 2. measuring and recording thepower of the 0th order beam,
 2. measuring and recording the power of the0th order beam,
 3. measuring and recording the power of the selected oneof the +1st or -1st diffraction order,
 3. measuring and recording thepower of the +1st order beam,
 3. A means according to claim 2 whereinthe angle is approximately the Bragg angle.
 4. Means for dividing a beamof coherent radiation into a pair of beams having powers controlledaccording to a predetermined ratio, said means comprising: a diffractiongrating positioned to intercept said beam at an angle to placeessentially all the energy affected by said grating into the 0th and theselected one of the +1st and the -1st diffraction orders, monitoringmeans positioned to intercept the resulting beam in the unselected oneof said +1st or -1st diffraction orders to provide an output signalrepresenting the power of said resulting beam, and positioning meansresponsive to said output signal for rotating the said diffractiongrating to set the power of said unselected beam to a desired valuewhereby the power ratio of the 0th to the one other of said +1st and-1st diffraction orders is set to a desired value.
 4. calculating thesplit ratio of the power levels recorded in the steps 2 and 3, 4.measuring and recording the power of the -1st order beam, 5.repositioning the grating to a slightly different angle and repeatingsteps 2, 3 and 4,
 5. measuring and recording the angle of said gratingto the incident beam,
 5. In a system for dividing a beam of coherentradiation into a pair of beams having a desired power ratio, thecombination comprising: a holographic Bragg phase diffraction gratingpositioned to intercept said beam at approximately the Bragg angle toplace essentially all the non-absorbed energy affected by said gratinginto the 0th and the selected one of the +1st and -1st diffractionorders, means for rotating said grating to vary the angle at which thebeam is incident on the grating, indicating means, coupled to saidrotating means for representing the angular position of said grating,said indicating means having a scale calibrated according to the ratioof power of the 0th and the selected one of the +1st and -1stdiffraction orders.
 6. The method of calibrating a holographic Braggphase diffraction grating in terms of the split ratios of power betweenthe 0th diffraction order and the selected one of the +1st or -1stdiffraction orders comprising the steps of:
 6. repositioning thegratings to a slightly different angle and repeating steps 2 - 5 todevelop a table of split ratios for grating angles in the range of theBragg angle.
 6. repeating the preceding steps to provide sufficientmeasurements to calibrate the split ratio between the 0th order and theselected one of the +1st or -1st order in terms of the power of theunselected one of the +1st or the -1st order.
 7. The method ofcalibrating a holographic Bragg phase diffraction grating in terms ofthe power of the unselected one of the +1st or -1st diffraction orderand its relationship to the split ratio between the 0th diffractionorder and the selected one of the +1st or -1st diffraction orderscomprising the steps of:
 8. In an optical system for dividing a primarybeam of coherent radiation into a pair of beams, means for holding thepower of said primary beam to a constant value comprising: a diffractiongrating positioned to intercept said primary beam at an angle to placeessentially all the energy affected by the grating into the 0th and theselected one of the +1st and -1st diffraction orders, monitoring meanspositioned to intercept the resulting beam in the unselected one of said+1st or -1st diffraction orders to provide an output signal representingthe power of said beam, output power control means for varying the powerof said primary beam in response to a signal applied to a controlterminal, means connecting said output signal from said monitoring meansto said control terminal whereby a change in the power in said primarybeam results in a change in the output signal from said moNitoring meanswhich is effective to restore the beam to the original power levelthrough said control means.
 9. A system according to claim 5 whereinsaid output power control means includes a power supply for the sourceof the primary beam.
 10. A system according to claim 5 wherein saidoutput power control means includes a modulator positioned to interceptsaid primary beam.